Recently, along with the development of optoelectronics, demands for transparent polymers for optical uses having optically superb isotropy are increasing. Especially, optical films having optical properties applicable for phase films of liquid crystal displays and the like are strongly desired.
A polycarbonate resin obtained by reacting 2,2-bis(4-hydroxyphenyl)propane (generally referred to as “bisphenol A”) with phosgene or carbonic acid diester, especially a polycarbonate film formed of such a polycarbonate resin, is used for packaging, optical devices, display devices and other industrial uses. Recently, such a polycarbonate film is a target of attention as a material of phase plates, polarization plates, plastic substrates and the like of optoelectronics devices such as liquid crystal displays, and are now progressively put into practice. Especially for TFT liquid crystal displays remarkably advancing today, among the liquid crystal displays highly developed recently, a polycarbonate film is a target of attention as a phase film used between the liquid crystal layer and the polarization plate in order to improve the visibility of images.
Such a phase film has a role of converting elliptical polarization transmitted through the liquid crystal layer into linear polarization, and is formed of a monoaxially stretched film of a polycarbonate resin mainly formed of bisphenol A.
When used as a phase film, a film obtained by molding a polycarbonate resin formed of bisphenol A has a high photo-elastic coefficient because of the optical anisotropy of the benzene ring of the polycarbonate resin, which causes a problem that the phase difference variance is large due to the low stretching ratio. In addition, a film in a liquid crystal display needs to be treated at a high temperature of 180° C. or higher during the process of forming an alignment film or the like. The above-described film has a problem of not having a sufficient heat resistance.
As a polycarbonate resin having a high heat resistance and a low photo-elastic coefficient, a polycarbonate resin having a specific fluorene structure has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 6-25398 (Patent Document 1), Japanese Laid-Open Patent Publication No. 2001-253960 (Patent Document 2)).
Although having a high heat resistance and a low photo-elastic coefficient, a polycarbonate resin having such a structure may be ruptured when the film is stretched or coiled and may be weak against folding. A polycarbonate film weak against folding does not provide a smooth cutting face when being cut after coiled, and is not sufficiently strong and so may be ruptured when being stretched. Improvements on these points are desired.
A conventional aromatic polycarbonate resin has a problem that the high photo-elastic coefficient and the low fluidity thereof cause a large birefringence as a result of molecule alignment at the time of molding and residual stress. Therefore, for molding an optical material formed of such a conventional aromatic polycarbonate resin, a resin having a relatively low molecular weight is used in order to improve the fluidity, and the resin is molded at a high temperature in order to reduce the birefringence of the product formed using such a resin. However, with the conventional aromatic polycarbonate resin, there is a limit on the reduction of the birefringence even where the above-mentioned means is taken. Therefore, along with the recent spread of the fields in which optical materials are used, a material having a still lower photo-elastic coefficient and a still higher fluidity is strongly desired to be developed in a part of the optical material fields. For this purpose, attempts have been made to develop a resin having a small birefringence.
In the mean time, an optical material having a high refractive index realizes a lens element with a face having a small curvature, and so reduces the aberration of this face. This reduces the number of necessary lenses, the lens eccentricity sensitivity, and the lens thickness, and so reduces the size and weight of the lens system. In the field of lenses for glasses, the lens thickness can be reduced for the same power of glasses, which provides an advantage of improving the external appearance of the glasses.
As an optical resin having a high refractive index and a small birefringence, a totally aromatic polycarbonate resin copolymer using bisphenol A with a fluorene structure having a high polarization ratio in a side chain direction has been studied (see Japanese Laid-Open Patent Publication No. 6-25398 (Patent Document 1), Japanese Laid-Open Patent Publication No. 7-109342 (Patent Document 3)).
A homopolycarbonate resin of an etherdiol with a fluorene structure having a high polarization ratio in a side chain direction and a phenol skeleton in a straight chain direction, and a copolymer of such a homopolycarbonate resin and a bisphenol have been shown (see Japanese Laid-Open Patent Publication No. 10-101787 (Patent Document 4), Japanese Laid-Open Patent Publication No. 10-101786 (Patent Document 5)).
A copolymer of a bisphenol with a fluorene structure having a large polarization ratio in a side chain direction and tricyclodecane[5.2.1.02,6]dimethanol has been proposed (see Japanese Laid-Open Patent Publication No. 2000-169573 (Patent Document 6)).
As described above, various types of materials having a small birefringence have been developed. Nonetheless, these technologies have problems that the refractive index is low, the moldability and the heat resistance are insufficient to provide a satisfying product, and the materials are colored.
Acenaphthene bisphenol, which is a monomer, is known and is used only as a resist composition. A resin having such a skeleton or properties thereof are not known at all, and there is no example in which such a resin is used for a specific use (see Japanese Laid-Open Patent Publication No. 2005-326838 (Patent Document 7), Japanese Laid-Open Patent Publication No. 2005-346024 (Patent Document 8)).    Patent Document 1: Japanese Laid-Open Patent Publication No. 6-25398    Patent Document 2: Japanese Laid-Open Patent Publication No. 2001-253960    Patent Document 3: Japanese Laid-Open Patent Publication No. 7-109342    Patent Document 4: Japanese Laid-Open Patent Publication No. 10-101787    Patent Document 5: Japanese Laid-Open Patent Publication No. 10-101786    Patent Document 6: Japanese Laid-Open Patent Publication No. 2000-169573    Patent Document 7: Japanese Laid-Open Patent Publication No. 2005-326838    Patent Document 8: Japanese Laid-Open Patent Publication No. 2005-346024